1. Field of the Invention
This invention relates in general to television receivers and in particular to a television receiver for receiving both 525-line interlaced video signals and 1050-line interlaced video signals.
2. Description of the Prior Art
The standard television system established by the National Television Systems Committee (NTSC) is a 525-line interlaced system. In a search for higher resolution, various other television systems have been developed including the 525-line progressive system and the 1050-line interlaced system. The 525-line progressive and the 1050-line interlaced systems are high definition systems which require signals having the standards issued by the Society of Motion Picture and Television Engineers as described in their publication entitled "Proposed American National Standard for Television Signal Parameters 1050/59.94/2:1 and 525/59.94/1:1 High Definition Production Systems", hereby incorporated by reference.
Because of the different types of systems available, it is desirable to have televisions which can operate with a variety of signals. U.S. Pat. No. 5,239,377 is a device for deriving a standard television signal from an interlaced high definition signal for display on a standard television receiver. U.S. Pat. No. 5,239,377, however, does not enable the television receiver to display both high definition signals and standard television signals. The differences between the NTSC, 525-line progressive and 1050-line interlaced make it difficult to create a television receiver which is operable for more than one type of signal.
As described in "Television How it Works" by J. Richard Johnson 2nd ed. 1956, also incorporated by reference, a 525-line interlaced signal includes (i) two fields each having 262.5 scan lines, and (ii) vertical and horizontal sync signals. (It should be noted that other conventional systems operate in the same manner as the NTSC system but the number of scan lines may vary). An electron beam scans each field on a picture tube in 1/60th of a second from left to right forming horizontal scan lines which progress from the top of the picture tube to the bottom of the picture tube. A horizontal sync signal indicates the end of a horizontal scan line. Upon detection of a horizontal sync signal the electron beam begins a new scan line which is oriented on the picture tube below the previous scan line. To begin a new scan line, the electron beam performs a horizontal retrace as shown in FIG. 1a and described below.
The vertical sync signal indicates the end of a field, or in other words, when the required number of horizontal scan lines have been completed. Upon detection of a vertical sync signal the electron beam returns to the top of the picture tube (a vertical retrace) to begin scanning a new field. The vertical retrace is not instantaneous and depending on the system can take anywhere from 10 to approximately 21 scan lines of the 262.5 scan lines. The vertical spacing between consecutive horizontal scan lines in a field is caused by the vertical deflection yoke deflecting the electron beam during a horizontal scan according to a vertical ramp signal. The vertical ramp signal is synchronized by an oscillator which is synchronized to the vertical sync signal. The timing of the oscillator is controlled by the sync pulse. Because this vertical ramp signal is applied to the vertical deflection yoke during scanning of the horizontal scan lines, the horizontal scan lines are slightly angled as shown in FIG. 1a. The voltage level of the vertical ramp signal causes a corresponding ramp-like current in the vertical deflection yoke. The amount of current in the vertical deflection yoke corresponds to the vertical position, on the picture tube, of the horizontal scan lines during scanning. Although the vertical deflection yoke can be coupled so that positive yoke current will either deflect the electron beam upwards or downwards, for ease of description it will be assumed that positive current in the vertical deflection yoke, produces upward deflection. Thus at the beginning of a field the positive current in the vertical deflection yoke causes the electron beam to be deflected to the top of the picture tube. It will also be assumed that the voltage level, and corresponding vertical yoke current decreases in a ramp-like fashion during scanning of each field (although depending on surrounding circuitry and the positioning of the vertical deflection yoke, the voltage level may increase in some systems during scanning of each field). This ramp signal is synchronized to the vertical sync signal such that a new ramp signal is generated each time a field has been completely scanned. The synchronization of the vertical and horizontal sync signals and the associated vertical ramp signals cause the electron beam to scan each successive horizontal scan line displaced from the preceding horizontal scan line by a predetermined amount, in the vertical direction, according to the ramp-like change in the vertical deflection yoke current, until all scan lines of each field are completed.
FIG. 1a shows how the two fields are interlaced in a 525-line-type interlaced system (NTSC). This same type of interlacing is also used in any conventional interlaced system having a non-integral number of scan lines in each field (hereinafter referred to as a conventional system or conventional signal). The open lines with solid arrows represent field 1 scan lines. The solid lines with solid arrows represent field 2 scan lines. The dashed lines with open arrows are the horizontal retraces for field 1. The dashed lines with solid arrows are the horizontal retraces for field 2. The dashed and dotted line with open arrows is the vertical retrace after a field 1 scan has been completed. The dashed and dotted line with solid arrows is the vertical retrace after a field 2 scan has been completed.
The scanning of an NTSC system begins at the beginning of, for example line 1, and scans the first scan line until a horizontal sync signal is detected. Once the horizontal sync signal is detected the electron beam performs a horizontal retrace, that is, no video signal is scanned and the electron beam positions itself at the beginning of the next horizontal scan line. As discussed above, the vertical position of each of the horizontal scan lines is determined by the value of the vertical ramp signal, i.e. the amount of current through the vertical deflection yoke at each point in time during a horizontal scan. Scanning is continued until 241.5 lines are scanned. (This assumes that a vertical retrace requires the amount of time corresponding to the scan time for 21 scan lines.) During scanning of the 242nd line, i.e. at a non-integral line number of 241.5, a vertical sync signal will be detected and the electron beam will traverse back to the top of the picture tube (execute a vertical retrace), during the next 21 scan lines, to begin scanning a new field. Since the electron beam is in the middle of a horizontal scan line when a vertical sync signal is detected, e.g. 241.5, it will begin scanning the first scan line of the second field at the middle M of the scan line. The middle M of the first scan line of the second field is at the same point, vertically, as the beginning B of the first scan line of the first field as these points correspond to the same current level in the vertical deflection yoke, i.e. the same current level induced by the vertical ramp signal. Therefore, due to the non-integral number of scan lines in each field, the scanning of the horizontal scan lines of the second field are now offset vertically from the respective horizontal scan lines of the first field by a distance equal to one-half the distance between two consecutive scan lines of the same field which causes the scan lines of the second field to be scanned between the scan lines of the first field creating a "naturally" interlaced picture as shown in FIG. 1a. This offset will be hereinafter referred to as the one-half line offset or deflection etc. The last line of the second field, e.g. the 242nd line, is scanned to completion since the scanning of the second field began in the middle of the first scan line.
The 525-line progressive system, which is shown in FIG. 1b, does not have two fields but rather only one field having 525 scan lines, a sample of which is shown as lines A-J in FIG. 1b. The 525 lines are scanned progressively within 1/60th of a second. There is no interlacing.
In the 1050-line interlaced system, there are 525 lines in a first field and 525 lines in a second field and each field is scanned in 1/60th of a second. Again, the vertical retrace requires anywhere from approximately 10 to approximately 21 scan lines of the 525 scan lines. Since each field has an integral number of scan lines, the "natural" interlacing does not occur, that is, each field is scanned beginning at the beginning of a scan line and ending at the end of a scan line. Because there is no "natural" interlacing in the 1050-line interlaced system, if the 1050-line interlaced signal is used with a conventional display system such as an NTSC (525-interlaced) system, the second field of 525 scan lines would be scanned directly over the first field of 525 scan lines.